#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii ;
typedef pair<ll, ll> pll ;
typedef vector<pii> vii ;
typedef vector<int> veci ;
typedef vector<pll> vll ;
typedef vector<ll> vecll;
// find_by_order order_of_key
#pragma GCC optimize("O3,unroll-loops")
//#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
#define F first
#define S second
#define pb push_back
#define endl '\n'
#define Mp make_pair
#define all(x) x.begin(), x.end()
#define debug(x) cerr << #x << " = " << x << endl
#define set_dec(x) cout << fixed << setprecision(x);
#define fast_io ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define file_io freopen("in.txt" , "r" , stdin) ; freopen("out.txt" , "w" , stdout);
#define lb lower_bound
#define ub upper_bound
#define for1(n) for(int i=1;i<=n;i++)
#define for0(n) for(int i=0;i<n;i++)
#define forn(n) for(int i=n;i>0;i--)
#define pq priority_queue <pii, vector<pii>, greater<pii>>
const int N=1e5+4,L=19,NN=N*L;
int A[N],n,m,k,q,dp[N][L][L],mn[N][L],dpp[N];
int main(){
fast_io
cin>>n;
for1(n){cin>>A[i];mn[i][0]=A[i];if(A[i]>=L)return cout<<"-1\n",0;}
for(int j=1;j<L;j++)for1(n)if(i+(1<<j)<=n+1)mn[i][j]=min(mn[i][j-1],mn[i+(1<<(j-1))][j-1]);
for1(n){
for(int j=0;j<L;j++)
dp[i][0][j]=NN;
dp[i][0][A[i]]=0;
if(A[i]!=0)
dp[i][0][A[i]-1]=1;
}
for(short int d=1;d<L;d++){
for1(n){
if(i+(1<<d)>n+1)break;
for(short int x=0;x<L;x++){
if(mn[i][d]<x){dp[i][d][x]=NN;continue;}
dp[i][d][x]=min(NN,min(dp[i][d-1][x]+dp[i+(1<<(d-1))][d-1][x],dp[i][d-1][x+1]+dp[i+(1<<(d-1))][d-1][x+1]+1));
}
}
}
dpp[0]=0;
for1(n){
dpp[i]=NN;
for(int j=0;j<L;j++)if(i-(1<<j)>=0)
dpp[i]=min(dpp[i],dpp[i-(1<<j)]+dp[i-(1<<j)+1][j][0]);
}
cout<<(dpp[n]==NN ? -1:dpp[n])<<endl;
}
